1. Field of the Invention
The invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD having a coating-type polarizer on its lower substrate.
2. Description of the Related Art
As the information-oriented society develops rapidly, there has recently emerged a demand for a flat panel display device having excellent characteristics such as a slim profile, light weight, and low power consumption.
An LCD, which is one of flat panel display devices, generally has excellent visibility, low power consumption and low heating value compared with a cathode ray tube (CRT) device of the same size.
Accordingly, the LCDs are now drawing attention as optimal next-generation display devices for cellular phones, computer monitors, and televisions, together with plasma display panels (PDPs) and field emission displays (FEDs).
An LCD includes two substrates having electric field generating electrodes. Surfaces of the two substrates, on which the two electrodes are formed, face each other. Liquid crystal molecules are injected between the two substrates. Applying a voltage between the two electrodes generates an electric field. The liquid crystal molecules reorient due to the electric field, and the light transmittance varies depending on the movement of the liquid crystal molecules. In this manner, the LCD displays images.
Generally, liquid crystal molecules have anisotropy. The anisotropy of a liquid crystal cell containing the liquid crystal molecules is varied depending on the distribution and tilt angle of the liquid crystal molecules with respect to the substrate.
This anisotropic property is an important factor in changing the polarization of light depending on the viewing angle of the liquid crystal cell.
FIG. 1 schematically illustrates the structure of a related art LCD.
As shown in FIG. 1, the LCD includes a lower substrate 120 having a thin film transistor (TFT), and an upper substrate 130 has a color filter. A liquid crystal layer 140 is interposed between the upper and lower substrates 130 and 120.
Also, the LCD further includes a first polarizer 129 attached to the rear surface of the lower substrate 120 and a second polarizer 139 attached to the front surface of the upper substrate 130. The first polarizer 129 polarizes natural light into linearly polarized light and transmits the same. The second polarizer 139 has a transmission axis perpendicular to the first polarizer 129.
The LCD further includes a backlight unit 110 provided at a lower portion of a liquid crystal panel, which includes the lower and upper substrates 120 and 130 and the liquid crystal layer 140. The backlight unit 110 supplies light emitted from a light source 111.
The lower substrate 120 includes a gate line and a data line perpendicularly arranged on a transparent substrate 121. Gate electrodes 122 extend from the gate line, and a gate insulating layer 123 is formed on an entire surface including the gate electrode 122. A semiconductor layer 124 is formed on the gate insulating layer 123, and a thin film transistor (TFT) including source/drain electrodes 125a and 125b formed on a semiconductor layer 124, is formed on the lower substrate 120.
A pixel electrode 127 connects with the drain electrode 125b of the TFT through a contact hole formed in a passivation layer 126 formed on the lower substrate 120.
Further, on the upper substrate 130, a black matrix 132 prevents light from being transmitted to regions except the pixel electrode 127, and the black matrix 132 is formed on a transparent substrate 131. A color filter pattern 133 of red (R), green (G), and blue (B), for reproducing a color is formed on the black matrix 132, and a common electrode 134 is formed on the color filter pattern 133.
The first and the second polarizers 129 and 139 are also respectively formed on outer surfaces of the lower and upper substrates 120 and 130, so that their transmission axes are perpendicular to each other.
Polarizers 129 and 139 separate natural light into polarized light components, pass a specific polarized light component among the polarized light components and absorb or disperse other components.
Light has the properties of electromagnetic waves and has vibrational components whose directions are perpendicular to the lights incident direction. Polarized light means a light beam having polarization in a specific direction, namely, light having a strong vibrational component in a specific direction among the vibration components perpendicular to the incident direction of the light.
Therefore, the vibration directions of light from the backlight unit 110 provided at the lower portion of the liquid crystal panel have the same probability for all directions perpendicular to the progression direction of the incident light.
At this point, the first and second polarizers 129 and 139 only transmit light vibrating in the same direction as their polarization axes, as compared to light vibrating in all directions. The polarizers therefore absorb or reflect light vibrating in other directions using an appropriate medium, thereby creating light vibrating in one specific direction.
The first and second related art polarizers 129 and 139 are arranged on opposite sides of the liquid crystal layer 140, and they attached to the lower and upper substrates 120 and 130 in such a way that their polarization axes may be perpendicular to each other. Therefore, when light passes through the liquid crystal layer 140, the intensity of a transmitted light is controlled depending on rotation degree of the polarization axes, such that gray-scale reproduction between black and white is possible.
However, in the related art structure, part of the polarized light transmitted from the first polarizer 129 attached to the lower substrate 120 of the liquid crystal panel changes into non-polarized light while passing through the liquid crystal panel.
That is, light is scattered by a stepped portion formed on the lower substrate 120 and the color filter layer 133 formed on the upper substrate 130, so that part of the polarized light transmitted from the first polarizer 129 changes into non-polarized light.
As described above, non-polarized light generates in the interior of the liquid crystal panel, and the light transmittance of the related art LCD is thus lowered and the contrast ratio deteriorates.